Known current sensors operating in accordance with the principle of compensation are often also called closed loop current sensors. Closed loop current sensors are based on a magnetic circuit, called core, of a highly permeable material which encloses a primary winding with the current that is to be measured. A flux sensor element located in the magnetic circuit, e.g. in an air gap, will detect any magnetic flux induced in this circuit and will generate a proportional signal. This signal is amplified by some electronic power stage, called booster circuit, which will generate a current through a secondary winding. This current is opposed to the primary current, establishing a negative feedback, and it will compensate the effect on the magnetic circuit, apart from a small magnetic induction which is specified as the actuating variable for the operation of the feedback loop. This residual induction corresponds to a current error of the whole sensor and needs to be kept small. This can be achieved by designing the amplifier for a very high gain, in terms of secondary current per magnetic induction of the core.
In many known implementations, closed loop current sensors are equipped with linear amplifiers that continuously produce high conduction losses in their transistors or operational amplifiers, for example in the range of medium amplitudes. These semiconductor losses will contribute to the total losses and the supply power demand of the sensor. Moreover, they may give rise to local heating in the sensor, leading to either reduced reliability or increased design effort in terms of cooling means and component and sensor size and cost.
The prior arrow art show a manner of reducing conduction losses in the booster circuit by using a switched mode amplifier with a pulse width modulation scheme instead of a linear amplifier. By continuously switching between full conduction and insulation, these types of devices significantly reduce the conduction losses, even though at the expense of some additional switching losses. Total losses will be usually still much lower as compared to linear amplifiers. A switched mode amplifier will generate a pulsed output voltage whose average will correspond to the output of a corresponding linear amplifier. A continuous output can be restored by means of appropriate filtering.
A closed loop current sensor with a switched mode amplifier is described in DEOS196 42 472, where the sensor uses a switchable booster for decreasing the power specification for the compensation current, and for reducing the losses at an operation with excessive supply voltage, and which is controlled by a pulse width modulated gating signal, which possesses a duty cycle depending on the measured value.
U.S. Pat. No. 6,713,999 B1 shows a current sensor which is provided with low pass filters for stabilizing the pulse width modulated compensation signal, where the current sensor is also provided with an additional RC element, and furthermore, a limiting means including (e.g., consisting of) Zener diodes and an ohmic resistance is provided for suppressing fast current transients.
The pulse width modulation scheme provided in prior art systems uses a constant switching frequency, i.e. it is featuring constant switching losses, while the conduction losses are proportional to the secondary current. This leads to considerable semiconductor losses at high current levels and results in some design limitations such as size and cost of the electronic components. Another effect coming with the pulse width modulation scheme is related to a ripple caused by the constant switching frequency. The absolute value of the ripple is almost independent of the current value, meaning that the relative ripple value at small currents may become quite high. Elaborated filters should then be specified to reduce the ripple.